1. Field of the Invention
This invention relates to an improved process for the preparation of polyimides and is more particularly concerned with an improved process for the isolation of non-cellular polyimides in a solid state.
2. Description of the Prior Art
Non-cellular polymides are widely used for wire coating, lamination and related adhesive uses in environments which require high resistance to deformation by heat and to combustion. They have been prepared in a two step process by reaction of a polyamine with a polycarboxylic polyanhydride to obtain the corresponding amide-acid intermediate. The latter amide-acid is then cyclized by dehydration to form the desired polyimide. Illustrative of this method are the procedures described in U.S. Pat. Nos. 3,179,630 and 3,179,631 and by Scroog et al., J. Polymer Science, Part A, Vol. 3, pages 1373 to 1390, 1965.
More recently, a process has been devised in which a polymide is obtained directly by interaction of a polyisocyanate and a polycarboxylic polyanhydride; see, for example, U.S. Pat. No. 3,562,189. In this method the polyisocyanate and polyanhydride are brought together in the presence of an aprotic solvent such as dimethylsulfoxide, N-methylpyrrolidone and the like and heated, if desired. When the reaction is carried out on a relatively small scale, e.g. in a laboratory scale preparation, the polyimide generally separates from solution in the form of a solid or alternatively, the polyimide can be precipitated from the solution of reaction product by addition of a second solvent. However, when this reaction is carried out on a larger scale, i.e. to prepare batches of material of the order of several kilograms or higher, it is found to be extremely difficult to isolate the polyimide from the reaction product.
Presumably because the polyimide reaction product is a mixture of materials of differernt molecular weights, the reaction product tends to separate from the reaction mixture, either spontaneously or after the addition of solvent in which the polyimide is insoluble, as a viscous sticky mass. The latter is very hard to handle with conventional equipment and greatly complicates the problem of production of large quantitles of the desired polyimides.
We have now found that this problem can be solved in an elegant and economical manner by a procedure which is described hereinafter. This procedure can be applied to isolation of polyimides when prepared by any of the procedures known in the art.